Digital signal synthesizer
First Claim
1. An apparatus for synthesizing a designated waveform having a range of frequencies the upper end of which is selectable up to an analysis frequency, wherein the waveform is synthesized from a digital representation of a base level signal sampled at a predetermined sample rate and further wherein each of the samples is taken with a predetermined resolution, comprisingmeans operating at the predetermined resolution and at a fixed conversion rate for converting a digital representation of a signal, presented at the conversion rate, into analog form;
- digital filter means for interpolating the digital representation of the base level signal to have an interpolated sample rate corresponding to the conversion rate without modifying the resolution of the samples and simultaneously achieving flat pass band ripple and high stop band attenuation, wherein the interpolated digital representation of the base level signal is supplied directly to the converting means for conversion into analog form; and
filter means, having a cutoff frequency which is fixed at a frequency corresponding to the analysis frequency and coupled to the converting means, for suppressing frequencies above the analysis frequency.
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Abstract
A method and apparatus for converting a digital signal into a band-limited analog signal with variable bandwidths, a high level of performance, high spurious frequency rejection, and simple implementation, employing polyphase interpolating digital filters, a fixed-sampling rate digital to analog converter, and a fixed-frequency analog low-pass filter.
30 Citations
22 Claims
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1. An apparatus for synthesizing a designated waveform having a range of frequencies the upper end of which is selectable up to an analysis frequency, wherein the waveform is synthesized from a digital representation of a base level signal sampled at a predetermined sample rate and further wherein each of the samples is taken with a predetermined resolution, comprising
means operating at the predetermined resolution and at a fixed conversion rate for converting a digital representation of a signal, presented at the conversion rate, into analog form; -
digital filter means for interpolating the digital representation of the base level signal to have an interpolated sample rate corresponding to the conversion rate without modifying the resolution of the samples and simultaneously achieving flat pass band ripple and high stop band attenuation, wherein the interpolated digital representation of the base level signal is supplied directly to the converting means for conversion into analog form; and filter means, having a cutoff frequency which is fixed at a frequency corresponding to the analysis frequency and coupled to the converting means, for suppressing frequencies above the analysis frequency. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15)
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16. An apparatus for synthesizing a designated signal having a range of frequencies the upper end of which is selectable up to an analysis frequency, comprising
dedicated memory means for storing a digital representation of a base level signal, wherein the digital representation of the base level signal has a predetermined sample rate and each sample has a predetermined sample resolution; -
means operating at the predetermined sample resolution and at a fixed conversion rate for converting a digital representation of a signal into analog form; digital filter means coupled to the dedicated memory means for expanding the sample rate of the digital representation of the base level signal up to the conversion rate and for supplying the expanded digital representation of the base level signal directly to the converting means, wherein the amount of expansion is determined by dividing the conversion rate by the upper end of the signal range; and filter means, having a cutoff frequency which is fixed at a frequency corresponding to the analysis frequency and coupled to the converting means, for suppressing frequencies above the analysis frequency.
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17. An interpolating digital filter structure for implementing an elliptic filter function which is expressed in z-transform form as having poles and zeros comprising
a first section implementing all of the zeros of the transfer function comprising a polyphase implementation of a finite impulse response form having a plurality of parallel signal paths, and further wherein each path implements a selected non overlapping subset of the finite impulse response coefficients, where the coefficients are a function of the zeros of the filter transfer function; - and
a second section, cascaded with the first section, implementing all of the poles of the transfer function. - View Dependent Claims (18)
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19. An interpolating digital filter which filter interpolates by a designated primary interpolating factor comprising
a cascaded plurality of digital elliptic filters, each interpolating by a designated interpolation factor, wherein the product of the designated interpolation factors of the cascaded digital elliptic filters equals the primary interpolation factor, and further wherein each of the cascaded plurality of digital elliptic filters are simple interpolator sections having similar pass band/stop band transition characteristics.
Specification